Democrats kept the Senate Tuesday night, as one race after another fell like dominoes for the party and ensured its control of the chamber for another two years.

In two of the biggest wins, Democrats swiped away prized GOP seats in Indiana and Massachusetts. Then in Virginia, Democrat Tim Kaine disposed of Republican George Allen.

And the possible cherry on top? Heidi Heitkamp, North Dakota’s affable former attorney general, clung to a lead of just a few thousands votes over Rep. Rick Berg, leaving her on the cusp of an astounding upset.

Even Montana Sen. Jon Tester — one of the most vulnerable Democratic incumbents — jumped to an early lead against Rep. Denny Rehberg as votes were tallied early Wednesday morning.

While Republican Sen. Dean Heller held off Democratic Rep. Shelley Berkley in Nevada, Democrats have actually padded their 53-seat majority — a feat that was unthinkable when the cycle began.

“The Senate’s a series of failures for Republicans,” said Republican Ari Fleischer on CNN.

Even National Republican Senatorial Committee chairman John Cornyn didn’t attempt to sugarcoat the thumping.

“It’s clear that with our losses in the Presidential race, and a number of key Senate races, we have a period of reflection and recalibration ahead for the Republican Party. While some will want to blame one wing of the party over the other, the reality is candidates from all corners of our GOP lost tonight. Clearly we have work to do in the weeks and months ahead,” Cornyn said in a statement.

He’s referring, in part, to early Democratic victories in Florida, Connecticut, Pennsylvania, Ohio and Wisconsin — all states where Republicans had invested enormous resources to contest.

But the biggest surprises were Rep. Joe Donnelly and, if the results hold, Heitkamp.

The three-term Blue Dog Democrat captured an Indiana Senate seat that has been in GOP hands for 36 years. Donnelly handily defeated state Treasurer Richard Mourdock, who fatally wounded himself in the closing weeks with his controversial statement during a debate about rape and abortion.

“We just pulled off the biggest upset victory of 2012,” Donnelly told supporters.

Mourdock ousted six-term Sen. Dick Lugar in a May primary by 20-points. Mourdock’s team predicted that GOP standard-bearer Mitt Romney’s margin would help pull their candidate over the finish line, but instead he drastically underperformed the entire ticket.

The loss is a stinging setback for Republicans and highlights the party’s problems with fielding candidates who are palatable to a general election audience. Majority Leader Harry Reid (D-Nev.) argued the results showed the price of GOP intransigence.

“This is what happens when your number one goal is to defeat the president and not work to get legislation done,” he said.

Still, a triumphant Reid also issued a call for bipartisanship moving forward.

“I look at the challenges we have about us, and I reach out to my Republican colleagues in the Senate and the House,” Reid said. “Let’s come together. We know what the issues are, let’s solve them.”

In North Dakota, Heitkamp’s likely victory showed that retail politics still matter in a state with just over 680,000 residents.

Heitkamp proved to be the best pure campaigner of the cycle. Even Mitt Romney, who carried the state by 19 points, apparently couldn’t save Berg.

Democrats also won their top takeaway target in Massachusetts, as liberal consumer advocate Elizabeth Warren ousted Sen. Scott Brown.

Warren spent the summer dogged by questions about her claim to Native American heritage and some in her own party complained about her professorial approach. But she was able to nationalize the race in an overwhelming Democratic state and peel away moderate, independent women who were crucial to Brown’s stunning 2010 ascendance.

Independent Angus King captured the open seat in Maine. While the popular former governor has declined to state which party he would caucus with, Democrats are confident he will ultimately align with them.

“We are very encouraged that when the night is over, Harry Reid will be the majority leader of the United States Senate going into the 113th Congress,” Democratic Senatorial Campaign Committee executive director Guy Cecil told POLITICO.

Earlier, Sen. Bill Nelson (D-Fla.) notched an easy win over four-term Rep. Connie Mack, who was massively outgunned on the airwaves and hobbled by persistent questions about his character. Ohio Sen. Sherrod Brown trounced Republican Josh Mandel, who struggled with high unfavorable ratings because of his early tenure as state treasurer. Former WWE CEO Linda McMahon devoted nearly another $50 million to another Senate bid, only to fall short to Rep. Chris Murphy in Connecticut.

Missouri Sen. Claire McCaskill — once thought to be the cycle’s most vulnerable incumbent — easily defeated Republican Rep. Todd Akin, who was disavowed by most of his party after his explosive comments about “legitimate rape” in August.

In Virginia, Allen did not even wait for the Associated Press to call the race before conceding to Kaine, who was propelled to victory by his strength in support in the northern part of the commonwealth.

Allen was attempting to reclaim the seat he lost in 2006 after he used a racial slur to disparage a Democratic tracker. Kaine, a former Democratic National Committee chairman, framed his campaign around post-partisanship and independence, in the mold of Democratic Sen. Mark Warner.

In Wisconsin, liberal Democratic Rep. Tammy Baldwin breezed by former Gov. Tommy Thompson to become the first openly gay senator in the country.

Going into the night, even Republican operatives openly acknowledged before the polls closed that the GOP would fall short of the three or four pickups needed to flip control, leaving the party to lick its wounds for the second consecutive cycle.

“The most likely outcome at this point seems to be Republicans picking up a single net seat, losing a single net seat, or holding steady by winning two to replace Scott Brown and Olympia Snowe,” wrote Republican pollster Chris Wilson in his pre-election memo.

That means the 2013 Senate is likely to look much like the current one, with Democrats holding a 53-47 edge.

Final polling in Montana, where a jaw-dropping $40 million was spent, showed GOP Rep. Denny Rehberg and Democratic Sen. Jon Tester running neck and neck.

“It is very close,” said a labor source on the ground in the state.

“I have no idea what will happen in Montana,” added a GOP operative.

Earlier in the night, Democratic Senatorial Campaign Committee Chairman Patty Murray (Wash.) was euphoric as she addressed a crowd of supporters at the Liaison Hotel on Capitol Hill.

“Some results are still coming in, we have a long night ahead of us, but we can now say with certainty we have defied the odds, we have won the night, and we will have a Democratic majority in the Senate,” Murray said. The Washington Democrat says voters “want Washington to work toward progress. They want common sense, they want compromise, not tea party tactics and budgets.”

“What we saw out there that worried me was all that [super PAC] money, like we had never seen before,” Murray added. Singling out Sheldon Adelson and the Koch brothers, two GOP mega-donors who dumped tens of millions of dollars into the presidential and Senate races, Murray blasted these tactics.